Electric connector



y 1957 K. M. HAMMELL 2,800,638

ELECTRIC CONNECTOR Filed June 12, 1955 lZ 6a 3 Kemper M flammell UnitedStates Patent ELECTRIC CONNECTOR Kemper M. Hammell, Harrisburg, Pa.,assignor to AMP Incorporated, a corporation of New Jersey ApplicationJune 12, 1953, Serial No. 361,205

Claims. (Cl. 339-276) This invention relates to electrical connectors ofthe type which are adapted to be crimped onto one or more conductors,and more particularly to such an electrical connector which isespecially suitable for crimping onto insulated, coated or dirtyconductors.

' Many types of wires which are commonly used for making electricalconnections have a layer of tough, elastic, insulating coating such asFormvar or Formex (polyv nyl formal or polyvinyl acetal). In addition,bare electrical wires often become dirty, or develop corrosion layers,such as oxides or sulphides, which effectively increase the contactresistance or even insulate the conductor. When it is desired to makeconnection to conductors having such external layers or films it isfirst necessary to pierce or scrape away the insulating coating in somemanner before an electrical contact of low ohmic resistancecan beachieved with the current-carrying portion of the wire.

A common procedure has been individually strip or abra'sively clean allsuch dirty or coated wires before attempting" to make connectionthereto, as by the use of wire cutters or similar implements. Forcertain types of wirecoatings, this approach is feasible, althoughtime-v consuming andexpensive. For other types of wire coatings, such ascertain hard and resilient insulation films, this method has proven tobe impractical because of the mechanical difficulties of completelystripping off the coating without damaging the conductor; this isparticularly true when relatively fine wire, such as the type generallyknown as magnet wire, is involved.

In the past, attempts have been made to provide solderl'e ss connectorshaving relatively sharp teeth or ridges which are designed to be forcedthrough a covering of insulation by the application of compressiveforce. However, tough coatings such as Formvar are nearly impenetrableto connectors of this type.

' Accordingly, it is an object of this invention to provide anelectrical connector of the type which is adapted to-be crimped orpressure-forged onto one or more wires, which is capable of partiallyshearing the wires during the crimping operation, and of jamming thesheared surfacesiri a sticking taper so as to create a permanent andstableelectrical contact with the conducting portion of the wires. Otherobjects and advantages will be partly obviousfrom and partly pointed outin, the following description of one embodiment of the inventionconsidered together with the accompanying drawings in which: Figure 1 isa cutaway plan view of a connector embodying the present invention andcrimped onto a stranded conductor; Figure 2 is an enlarged fragmentarycross-section view of a grooved inner surface of the connector of Figure1; Figure 3 is a plan of the connector in flat condition before rollingup to ferrule form; and

Figure 4 is an enlarged cross-section view of a modified groove'shownwith its coining die. 7 In Figure 1, a terminal connector generallyindicated at 2 and having a ferrule portion 4 is shown crimped 2,800,638Patented July 23, 1957 onto a stranded wire 6 of an insulated cable 8.The ring tongue 10 (shown in Figure 3) of the connector 2 isrepresentative of the various contact portions adapted for attachment toanother conductor, a binding post or other terminal device (not shown).Each of the wires 6 may be coated with oxide, sulphide, oil or wax orinsulating material from the insulation coating 9.

The cutaway view in Figure 1 shows a number of grooves 12, the edges ofwhich form an abrupt angle adapted to shear through layers on theperipheral strands of the wires 6 and into the metal of the strands tomake contact with the current-carrying portions of these strands duringa crimping operation wherein the ferrule portion 4 is forced inwardlyagainst the conductor under sufiiciently heavy pressure to deform themetals. The sides of the grooves are sloped with an angle of a stickingtaper for the metals concerned, so that when, during such a crimpingoperation the portions of the wire 6 adjacent the grooves 12 are shearedand driven into the grooves, as indicated in the drawings at 6A, thesheared surfaces are jammed in the taper with such pressure that thesurfaces are tightly. held and are tight against corrosion. The crimpingpressure will advantageously force the wire to bottom against thedeepest surfaces of the grooves and coin itself into every crevicethereof. Because of this bottoming effect, the shearing action isprevented from seriously weakening the wire after piercing the layer ofinsulation; any additional crimping after the wires reach bottom resultsonly in coining of the conductor material.

It has been found that the dimensions of the grooves may vary Withinsubstantial ranges while still allowing the grooves 12 to be effectivein shearing into and permanently gripping the wire 6. Referring now toFigure 2, it has been found, for example, that the width 11 of thegroove 12 between the shear edges is particularly important in achievinggood shearing action. The depth 13 of the grooves 12 and the breadth ofthe lands 15 between the grooves are also important, as describedhereinafter.

The effectiveness of the connection is a function both of the crimpingpressure and the width of the groove.

It is advantageous to provide a groove width which is great enough toafford a sufiicient shearing action at each edge with respect to theinsulating layer or coating, and yet sufficiently small that areasonably large number of grooves may be formed across the inner faceof the conductor ferrule, since the effectiveness of such a serratedconnector is enhanced by providing additional grooves each adapted tobite into and make effective mechanical and electrical connection to theconductor Wires.

For the commonly used electrical wires, the gap size between theshearing edges of the grooves will ordinarily fall within the range ofapproximately 0.010 to 0.020 inch. The width dimension of the grooves,for superior results, should approximate the strand diameter of theconductor to about 0.02 inch. With solid wires the width of the groovesmay be relatively less than for stranded wire, but in general it is thestrand diameter rather than overall wire gauge which controls thedimensions of the groove. Such a dimensional range, additionally, willallow a relatively large number of grooves to be cut into the inner faceof the usual types of solderless connector ferrules, which is anadvantage not only because of the multiplication of area of fresh metalcontact, but because it assures contact with more strands in amultistrand wire, as the spiral lay of a wire brings different strandsinto position to engage the shear edges in different areas.

There is, however, a limit to the number of grooves which can profitablybe crowded into a ferrule, as a substantial land is required between theshear edges of adjacent grooves in order to assure the desired highstrength connection of low and stable resistance. Thus with grooves.01-.02 inch in width, their spacing center to center should be at least.025-.030 inch, and in general the width of the supporting lands shouldnot be less than the width of the grooves.

Because of the inward slope of the side walls of the groove 12, theconductor material which is driven into the grooves by the crimpingpressure takes on a compressive stress, and thus by its own elasticitymaintains a tightly pressed contact against the side walls of thegroove. That is, the conductor material flowing into the grooveencounters .a continually-decreasing cross sectional area, so that thismaterial is compressed as it is forced into a region of diminishingvolume. As is apparent, the amount of compressive stress so built up,and more especially the tendency of the sheared portion ofthe wire tohold tightly to the sides of the groove, is determined partly by theslope given to the groove walls. To minimize the possibility ofcorrosion it is advantageous to provide for maximum compressive stresson the conductor portion between the sloping sides of the grooves, whilekeeping the angle small enough to assure a selfholding taper engagement.

A range of optimum taper angles according to this invention, whichsatisfies these requirements is from to 60. This angle should be smallerfor tougher layers surrounding the conductor, and may be larger withsoft or frangible coats. For example, when making shear contact througha layer of Formvar, which is extremely tough and resilient, it has beenfound that a very good taper angle is about 5". Alternatively, when itis desired to shear through a layer of copper sulphide formed on theouter surface of a conductor, a taper angle as large as 50 has beenfound satisfactory.

The depth of the groove (indicated in the drawing by the numeral 13)should be suflicient to assure that any insulating layer will be shearedthrough before the sheared portions of the wires 6 bottom in and fillthe grooves. This filling, which stops the shearing movement, shouldoccur shortly after completion of shearing through of the insulationlayer or coating, and before any of the wires 6 have been sheared to theextent of probable fracture.

In general ,the optimum depth is roughly one half the diameter of theindividual strands of the conductors. Thus, with a #14 AN Wire, whichhas 19 strands each of about .0145 inch diameter, a groove depth ofabout 0.006 inch has been found to be quite advantageous, whereas in aterminal designed to be crimped onto bared #l2 A. W. G. Wires havingstrand diameters ranging from 0.0159 to 0.0185 inch, a groove depth ofabout 0.007 to 0.008 inch has been found advantageous in providing thedesired connector characteristics outlined hereinabove. Terminals havingthese groove depths are not limited to use only with the particularstrand diameters indicated, such terminals being adaptable generally foruse with a broader range of conductor sizes while still providingsuperior results to those accomplished without this invention. The rangeof optimum depth dimensions will, in general, lie between approximately0.005 and 0.010 inch, with greater depths for enameled wire, andespecially poly vinyl methylal enamels.

Within the range of the groove dimensions and slope limitationsindicated herein, the grooves will shear cleanly through tough layers ofinsulation when the connector is crimped. Furthermore, such grooves willproduce a stable electrical connection that is permanently maintained bythe endwise compression of the sheared portion of the wire, and willcreate a solid contact without danger of cutting through the conductors.For the predominantly common applications involving relativelysmallsized bare wires one may advantageously use a groove width of about0.010 inch, a depth of about 0.005 inch, and a taper angle of about 50.

Figure 3 shows a solderless connector of the type described hereinaboveand in which the ferrule-forming portion 4 is in a flattened conditionbefore crimping onto a conductor. Cut transversely across theconductor-contacting surface of the ferrule 4 are a plurality ofsubstantially parallel grooves 12. The ear portions 3 and 5 of theferrule-forming portion 4 are adapted to be folded upwardly (as viewedin the drawing) and wrapped around one or more conductors in telescopingrelation thereto, and to be pressed inwardly against such conductorsunder heavy pressure for forming an intimate, cold-forged connectiontherewith. This folding and crimping operation may be performed by avariety of tools especially adapted for the purpose, such as are wellknown in the art of solderless connectors.

To assure good shearing action, it is necessary that the edges of thegroove come as near as possible to the very vertex of the dihedralangle. In constructing a connector the grooves may be formed bymachining or stamping, but the tools, whether cutting tools or coiningdies, should be as sharp as possible to assure efiicient shear edges. InFigure 4, I have shown, somewhat exaggerated and enlarged, a relievedcoining die to improve the sharpness of the corner which forms the shearedge. I In this draw ing, the coining die 20 is relieved along the baseat 22,

so that when the die is pressed against the fiat ferrule blank 24, theedges 26 of the groove 30 will be initially formed somewhat sharper thanthe required shear so as to provide a margin of safety.

Although my invention is not limited to any particular metal orcombination of metals, it is important for best results that the metalin the shearing edges should be harder than the metal of the wire whichis to be engaged. Dead soft copper is therefore too soft for mostpurposes. I have found that a quarter hard copper or brass (60-70 onRockwell l5T scale) works well on copper wire of 50 or less Rockwell 1SThardness. If the grooves are coined into the metal the shearing edgesmay be worked more severely, and therefore be harder, than other partsof the ferrule. Especially if the shearing edges are first made sharperthan the final shear angle, e. g. as shown in Figure 4, and thenflattened to give the precise shear angle, the extra working of themetal adjacent the shear edge has a desirable effect.

When a connector constructed in accordance with the present invention iscrimped onto a stranded conductor, it is found that the inner strandstend to form outwardly in the region of each groove. The serrationpattern is therefore actually apparent through the wire to the center ofthe stranding. This effect is due to the extrusion of copper in the wiredue to pressure exerted by the lands during the crimping, and thetemporary absence of such pressure in the areas of the grooves. As aconsequence, the outward pressure of the central strands tendspermanently to force the outer strands into the serration groovesthereby maintaining a keyed mechanical connection as well as anefficient electrical connection between the conductor and the walls ofthe groove.

Experience has shown that an extremely good corrosion resistantconnection of the type described is formed without necessity ofproducing as great a reduction in wire cross section as has beenrequired in the past for high quality connections. An importantby-product, electrically, of the strongly keyed mechanical connec tionresulting from the present invention is that the wires are preventedfrom sliding in the crimped connection when under severe stress, as whenthey are bent sharply near the ferrule. This feature gives bettercorrosion resistance and much more stable conductivity in theconnection.

Comparative tensile tests of standard commercial ferrule-type solderlessconnectors having the usual V type serrations and connectors differingonly in that the grooves were made in accordance with the presentinvention showed an improvement of approximately 45% in pulloutstrength.

Connectors embodying the present invention are advantageous not only foruse with stranded conductors but also with solid conductors, even thoseof relatively large size. For example, two or three solid conductors of0.060 inch diameter each coated with Formvar may be crimped solidlytogether in the ferrule of a connector of the type hereinabovedescribed, the groove width between the shearing edges being about 0.012inch wide, the groove depth being about 0.008 inch, and the taper angleof the groove being about 5. With such an arrangement, the groove edgesshear cleanly through the Formvar and produce a stable, non-corrosiveelectrical connection of low ohmic resistance between the conductors.Such a result had not been regarded as possible before the presentinvention.

Experience has shown that when relatively deep grooves are cut into theferrule near its mouth the wire 6 leading out of the terminal will beweakened against fatigue; this may result in damage to the conductorstrands, as by breaking an individual strand in the sheared region.

Electrical connectors in accordance with the present invention mayadvantageously be formed with serrations other than the transversegrooves shown in Figure 3. For example, good results may be obtained byserrating the inner face of a ferrule with a number of indentations, thesurfaces of each indentation forming a truncated cone or pyramid, thedimensions of which (i. e., width, depth, and slope angle) conform tothe pattern set forth hereinabove. The bases of such cones will normallylie in the plane of the inner face of the ferrule, so that thecross-sectional view of each indentation will be similar to that shownin Figure 2; the outer shearing edges of such a frusto-conicalindentation may, of course, be raised slightly, analogous to the showingin Figure 4, and for the purposes described with reference to thatfigure.

When it is desired to crimp onto a stranded wire wherein the conductorsare arranged in a substantial spiral with respect to the wire axis, itmay be advantageous to cut the grooves of Figure 3 at such an angle withrespect to the axis of the ferrule that they make perpendicular contactwith the conductor strands. That is, each of the grooves 12 may bearranged so that it intersects the spiral lay of the wire at rightangles, or nearly so, to provide a superior grip on the strands in adirection longitudinal of each strand, and thereby improve the pulloutstrength of the connector.

Alternatively, the grooves may be cut in a direction substantiallyparallel with the axis of the ferrule, and with dimensions variedgradually from a minimum nearer the mouth of the connector to a maximum,more remote so that the wire will be compressed into a sticking taper ina direction radial of the connector ferrule, and will be taper-wedged byany pull on the wire in a direction longitudinal of the ferrule.

Although certain specific embodiments of this invention and alternativeshave been described with particularity, it is desired to emphasize thatthese are neither limiting nor exhaustive of the invention. They havebeen included to assist those skilled in the art in applying theprinciples of this invention, recognizing that the various features ofthe invention may be modified or adapted to suit the requirements ofparticular applications.

1 claim:

1. An electrical connector adapted for crimping onto one or moreconductors, comprising a ferrule-forming portion having its innersurface formed with a plurality of grooves, the sides of each of saidgrooves forming surfaces each of which makes a sharp-edged intersectionwith said inner surface at an included angle more than 90 degrees andless than 120 degrees, said sides as viewed in a longitudinal sectionthrough said ferrule-forming portion extending along a straight line tothe bottom of the respective groove, the bottom of each of said groovesforming a surface having a substantial width relative to the depth ofthe groove.

2. An electrical connector adapted for crimping onto one or moreconductors, comprising a ferrule-forming portion having its innersurface formed with a plurality of grooves, the sides of each of saidgrooves forming surfaces each of which makes a sharp-edged intersectionwith said inner surface at an included angle more than degrees and lessthan degrees, the bottom of each of said grooves forming a surface thatis flat when viewed in a longitudinal section through saidferrule-forming portion, said bottom further having a substantial widthrelative to the depth of the groove.

3. An electrical connector adapted for crimping onto one or moreconductors, comprising a ferrule-forming portion having its innersurface formed with a plurality of grooves, the sides of each of saidgrooves forming surfaces each of which makes a sharp-edged intersectionwith said inner surface at an included angle more than 90 degrees andless than 120 degrees, the bottom of each of said grooves forming awell-defined distinct surface parallel to said inner surface of saidferrule-forming portion and having a substantial width relative to thedepth of the groove.

4. An electrical connector adapted for crimping into one or moreconductors, comprising a ferrule-forming portion having its innersurface formed with a plurality of grooves, the sides of each of saidgrooves forming surfaces each of which makes a sharp-edged intersect-ionwith said inner surface at an included angle more than 90 degrees andless than 120 degrees, said sides as viewed in a longitudinal sectionthrough said ferrule-forming portion extending along a straight line tothe bottom of the respective groove, the bottom of each of said groovescomprising a surface forming a straight line when viewed in saidlongitudinal section, said bottom having a substantial Width relative tothe depth of the groove.

5. An electrical connect-or adapted for crimping onto one or moreconductors, comprising a ferrule-forming portion having its innersurface formed with a plurality of grooves, the sides of each of saidgrooves forming surfaces each of which makes with said inner surface anincluded angle more than 90 degrees and less than 120 degrees, theregions of intersection between said side surfaces and said innersurface comprising groove edges which are slightly elevated with respectto said inner surface to form pairs of sharp transverse ridges forshearing insulating coatings on a conductor onto which said connector iscrimped, the bottom of each of said grooves forming a surface having asubstantial Width relative to the depth of the groove.

6. An electrical connector adapted for crimping onto one or moreconductors, comprising a ferrule-forming portion having its innersurface formed with a plurality of grooves, the sides of each of saidgrooves forming surfaces each of which makes with said inner surface anincluded angle more than 90 degrees and less than 120 degrees, saidsides as viewed in a longitudinal section through said ferrule-formingportion extending along a straight line to the bottom of the respectivegroove, the regions of intersection between said side surfaces and saidinner surface comprising sharp groove edges for shearing insulatingcoatings on a conductor onto which said connector is crimped, the bottomof each of said grooves forming a surf-ace having a substantial widthrelative to the depth of the groove, the depth of each of said groovesbeing approximately one-half the spacing between the edges of thecorresponding groove.

7. An electrical connector as set forth in claim 1 wherein saidferrule-forming portion comprises a flat metal blank rolled into tubularshape, said grooves being arranged in side-by-side parallel fashion witheach groove extending in the form of a closed ring around the interiorof said ferrule-forming portion.

8. An electrical connector as set forth in claim 7 wherein the planes ofthe rings formed by said grooves are per pendicular to the longitudinalaxis of said tubular ferruleforrning portion.

9. An electrical connector as set forth in claim 1 crimped onto aconductor positioned Within the interior of said ferrule-forming portionWith said grooves completely filled by portions of said conductor forcedinto said grooves, the sharp edges of said intersections being shearedinto the outer surfaces of said conductor portions to formfreshly-exposed conductor surfaces in said grooves and the sloping sidesof the grooves pre-ssure-wedging said sheared surfaces throughout asubstantial area there-by establishing low-resistance andcorrosion-resistant con-tact to said conductor.

10. An electrical connector as set forth in claim 9 crimped onto aconductor having a coating of relatively non-conductive material, thesharp edges of said intersections being sheared into said coating toform freshlyexposed conduct-or surfaces in said grooves and the slopingsides of the grooves pressure-wedging said sheared surfaces throughout asubstantial area thereby establishing low-resistance andcorrosion-resistant electrical contact to said conductor.

References Cited in the file of this patent UNITED STATES PATENTS;

1,169,642 Heeter J an. 25, 1916 2,259,261 Miller Oct. 14, 1941 2,327,650Klein Aug. 24, 1943 2,452,932 Johnson Nov. 2, 1948 2,604,508 Bergan July22, 1952 2,674,725 Buchanan April 6, 1954 2,685,076 Hoffman July 27,1954

